---
title: RAG Retrieval Compare
emoji: 🔍
colorFrom: indigo
colorTo: green
sdk: gradio
sdk_version: 6.17.3
app_file: app.py
pinned: false
short_description: BM25 vs Dense vs Hybrid vs Rerank, side by side, on ZeroGPU
---

# 🔍 Retrieval methods, side by side

A teaching demo for technical researchers learning vector-database fundamentals for RAG.
Given a query, it retrieves the top-k results from several methods and shows them side by
side over a corpus of ~4,000 Europe PMC abstracts (microRNA & disease):

1. **BM25** (lexical) — `rank-bm25`
2. **Dense** (vector) — exact cosine similarity over sentence-transformer embeddings
3. **Hybrid** — Reciprocal Rank Fusion (RRF, k=60) over the BM25 and dense rankings
4. **Reranked** (optional, on-demand button) — a **cross-encoder** that scores
   `(query, document)` pairs jointly and reorders the hybrid shortlist. It can't be
   precomputed, so it runs live and the UI **times it** to make the cost visible.

Two embedding models are shipped — **general** (`all-MiniLM-L6-v2`, 384-dim) and
**biomedical** (`pritamdeka/S-PubMedBert-MS-MARCO`, 768-dim) — and a radio switches which
one Dense, Hybrid and the plot use. BM25 is lexical, so it's unaffected by the switch.

Plus a **metadata filter** (year / journal) that restricts the candidate pool *before*
retrieval — the distinctly vector-DB feature — and a **UMAP scatter plot** for spatial
intuition, with connector lines from the query to the documents each method retrieved.
Click any result to expand its full abstract.

## Retrieve-then-rerank, and how it differs from RRF

- **RRF (Hybrid)** is a *fusion* of rankings — it only uses BM25's and Dense's rank
  positions, no model, essentially free. It improves **recall** by combining lexical +
  semantic signal.
- **The reranker** is a *second-stage precision* step. A **bi-encoder** (Dense) embeds
  query and document separately; a **cross-encoder** runs them through a transformer
  *together*, so it judges relevance far more accurately — but at one forward pass per
  candidate, so it only runs on a shortlist (here, the hybrid top-30). It can only reorder
  what stage 1 surfaced, so recall still matters.

## How it's built (read this before the talk)

- **Everything is precomputed offline** by `build_index.py`, which fetches the abstracts,
  embeds the corpus with **each** model, fits a UMAP per model, and tokenises for BM25.
  The artifacts in `./data/` are committed. The app **never** embeds the corpus or calls
  an external API at startup — it only embeds your *live query* (and, on demand, runs the
  reranker).
- **ZeroGPU:** GPU is allocated on demand only inside the `@spaces.GPU`-decorated query
  embedding and reranking functions. Every model is loaded on CPU at import; nothing
  touches CUDA at startup. ⚠️ **The first GPU call after idle has a cold start of a few
  seconds — pre-warm with a dummy query (and a dummy rerank) before presenting.**
- **Swappable models:** `config.py` holds the `EMBEDDING_MODELS` registry and
  `RERANKER_MODEL` — the single source of truth for both the offline build and the live
  app. Add/swap a model there and re-run `build_index.py`.

## Exact search, honestly

The corpus is tiny, so dense retrieval uses **exact** cosine similarity (a plain NumPy
dot product) — instant and correct. At production scale you'd reach for an **ANN index**
(e.g. HNSW) to keep search fast over millions of vectors. We deliberately don't show a
fake HNSW timing comparison here: at this scale the difference is invisible, so it would
mislead rather than teach.

## Teaching caveat on the plot

The plot uses **UMAP** for the 2D projection (it shows local cluster structure better
than PCA). Two honest caveats: UMAP warps global distances, and the live query is placed
by an *approximate* out-of-sample `transform`, so its position is only indicative.
Retrieved points may not be the visually-closest dots. **The ranked lists are
authoritative; the plot is only for intuition.**

## Rebuild the index locally

```bash
uv venv
uv pip install -r requirements.txt
uv run python build_index.py   # writes ./data/
```